Even if you remove the CMOS battery entirely, the system will still POST and display info on the screen. It isn't required for the system to function. It's only required to save settings & system info so the BIOS can boot faster next time around.

That's definitely an older system... if you want you can grab a flashlight and look through the 120mm fan, inspect the tops of the large caps you can see... see if any are bulging or leaking.